This invention relates to improved aircraft throttle control apparatus for the optimum operation of an aircraft, more particularly for a terrain following (TF) system.
Aircraft thrust or throttle control historically has not been integrated with terrain following flight path angle control systems. Throttle control has been a manual function performed by the pilot who makes throttle adjustments to maintain nominally constant speed or to avoid excessive speed buildup during dives or excessive fall off during climbs. Some terrain following aircraft (e.g., B-1) have automatically implemented speed or mach number hold systems which have been suggested for use during terrain following flight. The mach number or speed hold rationale constitutes the current practice in throttle control for terrain following. It is unsatisfactory in a number of respects. In a typical mach hold auto throttle mechanization the commanded thrust or throttle setting is formed by feedback of mach number error, mach number rate error, and flight path angle summed through appropriate gains. The command structure operates well under nominally level flight to control to a desired mach number. During terrain following flight, however, even over moderately hilly terrain the system tends to give min-to-max variations in thrust in synchronism with the hills and valleys, as a component of thrust is commanded to balance the projection of weight along the flight vector. This component can be quite large and can exceed the incremental thrust capabilities of the engines particularly in an aircraft with a low thrust to weight ratio. As a result, a system of this type can be frequently saturated during TF flight, exhibiting bang-bang type control which is deleterious to engine life and unnecessary in terms of terrain following requirements. A certain amount of speed variation can be tolerated while terrain following as long as speed fall off during climbs is not so great as to risk aircraft stall or speed buildup during dives is not excessive. The speed hold philosophy for TF fails to take proper account of the fact that constant speed may not be possible in a thrust-limited aircraft attempting to hug closely rugged terrain. Additionally, the only anticipation built into mach hold scheme is through flight path angle which specifies a need for a power setting or thrust level to prevent speed change but does not address the consequences in the event that the thrust level cannot be delivered by the engines. In the past this problem has been circumvented by establishing a priori climb and dive limits to be observed during terrain following flight which attempt to limit speed changes to an acceptable level. These limits, however, are quite small in aircraft with low thrust to weight ratios and terrain following is consequently restricted.
Heretofore terrain following command computers have been analog and not particularly suited to the handling of stored data required by the disclosed algorithm. Further, the question of throttle control has been generally avoided by the compromise suggested above, i.e., the specification of climb/dive limits.
An example of an analog control system for throttle control is shown in U.S. Pat. No. 3,908,934, by Schloeman which is operative for several modes from take-off, through cruising, to landing, but does not include terrain following. An example of a digital control system for an aircraft is shown in U.S. Pat. No. 3,940,094 by Kreiss et al for a Wing Sweep Control System.